1. Field of the Invention
This invention relates to butterfly valves and more particularly to improvements in the sealing structure of a butterfly valve.
2. Prior Art
In the butterfly valve prior art, the seal is created by the pressurized contact of the valve with the inner surface of a seat ring in a radial direction. For example, as shown in FIG. 7, butterfly valves have been constructed with the valve 3 thrusting against the inner surface of seat ring 2 to close the valve; or, as shown in FIG. 8, butterfly valves have been constructed with the valve 3 having a pressurized contact with the top of a circular projection 12 formed in the inner surface of the seat ring 2. However, in the structure of FIG. 7, the valve 3 illustrated in the one dot chain line fits the seat ring 2 before reaching the closed position illustrated by the solid line and the distance S between the two positions represents a disadvantage in that the valve 3 thrusts against the inner surface of the seat ring 2 and therefore a force works against the seat ring 2 whenever the valve is opened or closed. As a result, not only is the rotational torque required to open and close the butterfly valve larger but also the additional force damages the seat ring. In the structure of FIG. 8, the distance S is as small as possible by means of making a circular projection 12 in the inner surface of the seat ring 2 to make a pressurized contact with the circumference of the valve with the top of the circular projection 12. In such a structure, the sealing efficiency in the closed position is inferior to that of FIG. 7 because there is a fixed value of sealing pressure when the valve 3 makes pressurized contact with the circular projection 12.